1. Field of the Invention
The present invention relates to a superconducting device employing a superconductor. More particularly, it relates to a light-sensitive superconducting device which detects light or an electromagnetic wave and also a superconducting switching device which is switched by irradiation with light or an electromagnetic wave.
2. Description of the Related Art
As a light-sensitive device employing a superconductor, a microbridge type Josephson junction device those bridge portion is irradiated with light is described in IEEE Transactions on Magnetics, MAG-17, No. 1, January 1981, pp. 88-91. A microbridge type Josephson junction device employing a superconductor expressed by BaPb.sub.x Bi.sub.1-x O.sub.3, the bridge portion of which is irradiated with light, is described in the official gazette of Japanese Patent Application Laid-open No. 130182/1985. A grain-boundary Josephson junction type light detector which utilizes the grain-boundary Josephson junction type light detector which utilizes the grain-boundary Josephson junction of a polycrystalline film of BaPb.sub.x Bi.sub.1-x O.sub.3, is described in the official gazette of Japanese Patent Application Laid-open No. 65582/1985 In addition, the official gazette of Japanese Application Laid-open No. 141582/1983 describes a sandwich type Josephson device the tunnel barrier of which is irradiated with light, whereby the current or voltage change of the tunnel barrier is detected owing to the effect of quasiparticle injection into a superconductor. Any of these prior-art techniques detects light by exploiting the fact that, when irradiated with the light, the superconductor constituting the Josephson device is influenced by quasiparticles (electrons or holes) excited within this superconductor, resulting in the change of the characteristic of the Josephson device.
Any of the prior-art techniques, however, directly irradiates the superconductor with the light and therefore has the problem that the light cannot be detected at a high sensitivity due to a high light reflectivity. Another problem is that, since the detection sensitivity of any cf the devices lowers for the short wavelengths cf the incident light, the device is applicable to only specified wavelengths.
Among the prior-art techniques, the expedient described in the official gazette of Japanese Patent Application Laid-open No. 65582/1985 detects light by utilizing the fact that, when the polycrystalline film made of the oxide-superconductor material is irradiated with the light, the Josephson junction produced at the crystal grain boundary thereof generates a voltage. In this case, the voltages which are generated at such crystal grain boundaries are not always constant, but they are somewhat different in the respective devices. Therefore, the expedient has the problem that the devices of uniform characteristics are difficult to be fabricated.
As a light-sensitive device employing a superconductor and a semiconductor, a sandwich type Josephson device the tunnel portions of which are made cf a CdS film having pinholes is described in Physical Review Letters, Vol. 20, No. 23, pp. 1286-1289. This device forms tunnel junctions through the pinholes of the CdS film, and therefore has the problem that the characteristics of the junctions vary depending upon the numbers of the pinholes and are very difficult of control.
Further, this prior-art technique does not take into consideration the influence of a strain attributed to the thermal expansion of the superconductor as exerted on the characteristic of the device. More specifically, the strain in the material ascribable to a temperature cycle or a thermal shock induces the cracks or lattice defects of the material and affords a change to the characteristic of the device, to pose the problem that the operation of the device itself becomes unstable.